A flat panel display device has been widely applied for its many advantages such as a thin body, power saving, and radiation-free. Current flat panel display devices mainly include a Liquid Crystal Display (LCD), and an Organic Light Emitting Display (OLED). A Thin Film Transistor (TFT) is an important component of the flat panel display device, and may be formed on a glass substrate or a plastic substrate and is usually applied in for example LCD and OLED as a switching device and a driving device.
At present, the OLED display device is a mainly current-controlled light emitting device, and the luminance uniformity is controlled by a corresponding current. Usually, a thin film transistor array substrate of the OLED display device, corresponding to each sub-pixel, mainly includes a driving transistor that drives a light emitting diode, a switching transistor that transmits data voltage to a gate node of the driving transistor, and a storage capacitor that enables a certain level of voltage to maintain one frame time.
In the existing thin film transistor array substrate of the OLED display device, all thin film transistors, that is, the driving transistors and the switching transistors, adopt the same material of an active layer. Normally, poly-silicon or a metal oxide semiconductor material is adopted as the material of the active layer of the thin film transistor. The active layer of the thin film transistor which uses a poly-silicon material has high mobility, but with a large leakage current and worse uniformity; while the active layer of the thin film transistor which uses the metal oxide semiconductor material has better uniformity, but with comparatively low mobility. Therefore, in the thin film transistor array substrate of the OLED display device, for the driving transistor and the switching transistor that implement different functions, if they both use a single semiconductor material as the active layer, the disadvantage (bad uniformity or low mobility) of the semiconductor material will be enlarged, which is not conducive to improvement of the display quality of the OLED display device.